Electroluminescent device

ABSTRACT

An electroluminescent (EL) device achieves a significant increase in the lifetime of EL light sources by using a driver having a standard DC-to-AC inverter generating a high-frequency quasi-sinusoidal signal, and a generator of low-frequency rectangular pulses. The inverter and the generator are connected so that the quasi-sinusoidal signal is fed to the EL light source in packets. The duration of a packet is equal to the duration of the rectangular pulse. The duration of the rectangular pulses and of the packets remains constant during the entire operating cycle of the EL light source. The duration of intervals between rectangular pulses and between packets is variable, causing changes in the duty cycle, and is determined by the signal passing through a feedback circuit having an EL light source, a photosensitive element, an amplifier, and an input of the rectangular signal generator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electroluminescent devices, and morespecifically, to electroluminescent devices with enhanced lifetime.

2. Description of the Related Art

Conventionally, an electroluminescent (EL) light source comprises twocongruently disposed electrodes, one of them being transparent withinthe optical spectral region, with dielectric and electroluminescentlayers located in-between. Applying alternating voltage withcorresponding frequency and amplitude, for example, within the ranges of10² Hz–10⁴ Hz, 30 V–300 V, to the electrodes, the electroluminescentlayer starts emitting light throughout the transparent electrode. In thecourse of operation, degradation processes occur in the light-emittinglayer, which lead to a decrease in the brightness of the EL light sourceand a simultaneous growth of its impedance. Sometimes this process iscalled aging of the EL light source. The lifetime of the EL light sourceis defined as the time of continuous luminescence with the luminescencebrightness decreasing down to half of the initial brightness, known asthe half-decay time. The higher the frequency and the amplitude of theinput voltage, the higher is the initial brightness of the EL lightsource. However, in this case its lifetime is shorter than in the caseof applying input voltage with a lower frequency and amplitude.

Standard EL light source drivers are known and in use, which aredescribed, for instance, in the U.S. patents listed in Table 1:

TABLE 1 U.S. Pat. No. Issue Date Patentee Class/Subclass 3,749,977 Jul.31, 1973 Sliker 315/283 4,254,362 Mar. 3, 1981 Tulliners 315/2194,611,150 Sep. 9, 1986 Ball et al. 315/307 4,633,141 Dec. 30, 1986 Weber315/307with each of U.S. Pat. Nos. 3,749,977; 4,254,362; 4,611,150; and4,633,141 being incorporated herein by reference.

The devices disclosed in U.S. Pat. Nos. 3,749,977; 4,254,362; 4,611,150;and 4,633,141 are DC-to-AC inverters for converting DC voltage into ACvoltage with a specified frequency and amplitude, and with an outputsignal shape close to a sinusoidal shape. The transformer of theDC-to-AC inverter determines the output voltage and also plays a role indetermining the frequency. An EL light source together with a DC-to-ACinverter constitute a compensating integrated circuit. The capacitanceof the EL light source coupled with the inductance of the secondary coilof the transformer constitute a tunable LC circuit. The DC-to-ACinverter functions as a resonating oscillator compensating for thedecreasing capacitance of an aging EL light source. As the EL lightsource ages, the DC-to-AC inverter compensates for the decrease in theoutput brightness by increasing its output voltage and frequency. Theuse of DC-to-AC inverters described above increases the lifetime of ELlight sources by about 20%–30% in comparison with the lifetime of the ELlight source fed with an input AC voltage with frequency and amplitudeunchanging in time.

A much more efficient method of compensating for the processes of ELlight source aging was proposed in U.S. Pat. No. 5,440,208, issued Aug.8, 1995 to Uskali et al., in class/subclass 315/169; with U.S. Pat. No.5,440,208 being incorporated herein by reference.

According to U.S. Pat. No. 5,440,208, a driver is disclosed whichinvolves a feedback circuit comprising a voltage drop divider using aresistor connected in series with the EL light source. The voltage dropon this resistor is changed in the process of the EL light source aging.The voltage from this divider is fed to one of the inputs of a pulsegenerator, so that as the EL light source ages, pulse amplitudeincreases. A serious drawback of the proposed solution is that itinvolves a high initial dissipated power, and besides, the shape of thegenerated signal is far from being sinusoidal, which intensifies thedegradation processes in the EL light source.

Several types of drivers are known with a high-frequency sinusoidalsignal fed in packets. As a rule, such drivers are used to feedmulti-segment EL light sources, as described in the U.S. patents listedin Table 2:

TABLE 2 U.S. Pat. No. Issue Date Patentee Class/Subclass 4,238,793 Dec.9, 1980 Hochstrate 345/102 4,253,097 Feb. 24, 1981 Hochstrate 345/1024,449,075 May 15, 1984 D'Onofrio et al. 315/196with each of U.S. Pat. Nos. 4,238,793; 4,253,097; and 4,449,075 beingincorporated herein by reference.

FIG. 1 shows an electrical schematic diagram of one of several types ofavailable DC-to-AC inverters 10 for EL light sources in the prior art.If the selection of resistors 12, 14 and transistor 26 is correct, theinverter 10 generates a quasi-sinusoidal signal. The parameters of aresonant circuit 19, formed by a secondary inductance 22 of thetransformer 23 and capacitance of the EL light source 32, determine thefrequency of the output signal of DC-to-AC inverter 10. The turn ratioof the transformer 23 and the current generated in an auxiliary coil 28wound on the core 24 of the transformer 23 determine the amplitude ofthe signal generated by the DC-to-AC inverter 10. In the process of theEL light source operation, as a result of complicated physical processesof degradation or aging, its luminescence brightness decreases with asimultaneous increase in the impedance of the EL light source, mainlydue to the capacitance decrease. In this situation, the parameters ofthe oscillatory resonant circuit 19 and, to a lesser extent, the currentgenerated in the coil 28, are changed. Due to these changes, thefrequency and amplitude of the quasi-sinusoidal signal at the output ofthe DC-to-AC inverter 10 increase. This results in the increase ofluminescence brightness, which partly compensates for the brightnessdecrease caused by the light source aging. Respectively, the lifetime ofthe EL light source increases in comparison with the lifetime of asimilar EL light source fed by AC voltage with constant values of thesinusoidal signal frequency and amplitude. FIG. 9 shows a plot of thedependence of the luminescence brightness of the EL light source onlight emitting time for various types of drivers. Curve 78 relates tothe supply of AC voltage with parameters that remain unchanged in time,whereas curve 80, which relates to the supply of AC voltage using theDC-to-AC inverter 10 as shown by the schematic diagram, is presented inFIG. 1. Here the initial values of the EL light source brightness arethe same in both cases. It is evident that the EL light source lifetimein curve 80 is generally about 30% longer than the lifetime representedby curve 78.

BRIEF SUMMARY OF THE INVENTION

It is an objective of the present invention to achieve a significantincrease in the lifetime of EL light sources of any type in comparisonwith the lifetime of EL light sources operated by standard DC-to-ACinverters. According to the present invention, the above objective isachieved by feeding the EL light source using a driver which comprises,side-by-side, a standard DC-to-AC inverter generating a high-frequencyquasi-sinusoidal signal, and a generator of low-frequency rectangularpulses. The inverter and the generator are connected in such a way thatthe quasi-sinusoidal signal is fed to the EL light source in packets.

The duration of a packet is equal to the duration of the rectangularpulse. The duration of the rectangular pulses, and, respectively, thatof the packets of quasi-sinusoidal pulses, remains constant during theentire operating cycle of the EL light source. The duration of intervalsbetween rectangular pulses, and, respectively, between packets ofquasi-sinusoidal pulses is variable, causing changes in the duty cycle,and is determined by the signal passing through a feedback circuitcomprising:

-   -   a) an EL light source;    -   b) a photosensitive element, for example, a photoresistor,        optically connected to an EL light source;    -   c) an amplifier of the photosensitive element output; and    -   d) an input of the rectangular signal generator.

In the process of the EL light source operation, its brightness declinesdue to its aging, which leads to a change in the voltage drop across thephotosensitive element. This voltage change is amplified and fed to theinput of the rectangular-pulse generator, causing a decrease in theduration of the intervals between rectangular pulses. This is equivalentto a decreasing interval between the packets of high-frequency pulsessupplied to the EL light source. Thus, the duration of quasi-sinusoidalsignal fed to the EL light source increases, which substantiallycompensates for the brightness drop. Thus, the EL light source lifetimeincreases substantially.

To eliminate attenuation “tails” in the packets of high-frequencypulses, the output of the DC-to-AC inverter is shorted before the onsetof intervals of the low-frequency rectangular signal.

A capacitor with a respective capacitance may be connected in serieswith the EL light source. In this case, an increase in impedance and,respectively, a decrease in the capacitance of the EL light source leadto voltage redistribution. Thus, as the EL light source degrades, theeffective value of the voltage applied to it increases, which alsoresults in the compensation for the brightness loss and in the increasedlifetime.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Preferred embodiments of the invention are disclosed hereinbelow withreference to the drawings, wherein:

FIG. 1 shows an electrical schematic diagram of one of the widely usedtypes of standard DC-to-AC inverters for EL light sources in the priorart;

FIG. 2 shows a block-diagram of the EL light source driver circuit ofthe present invention;

FIGS. 3–5 show rectangular low-frequency signals at the beginning, inthe middle and at the end of the operating cycle of the EL light source,respectively;

FIGS. 6–8 show packets of quasi-sinusoidal pulses at the output ofDC-to-AC inverter representing the change in the duration of theinterval of the rectangular low-frequency signal modulating thehigh-frequency sinusoidal signal at the beginning, in the middle, and atthe end of the operating cycle of the EL light source, respectively;

FIG. 9 depicts the relation of brightness to the operation time foridentical EL light sources in case of standard drivers and drivers ofthe present invention; and

FIG. 10 shows the relation of intensity to the operation time forprototype EL light sources against conventional light sources.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a block-diagram of the driver 50 of the present inventionwith the EL light source 68 connected to it. The driver 50 shown in FIG.2 may be housed in a driver casing 60, and the driver 50 includes thefollowing components:

-   -   a) a DC-to-AC inverter 58 similar to that described above, which        generates a high-frequency quasi-sinusoidal signal;    -   b) a generator of pulse-width modulation of a high-frequency        quasi-sinusoidal signal comprising a voltage-controlled        generator 54 of rectangular pulses, control circuitry 56 of the        inverter 58, a logic inverter circuit 64, and auxiliary control        circuitry 62; and    -   c) a control unit for pulse-width modulation parameters having        an EL light source 68, a photosensitive element 72, an amplifier        52 of a feedback signal from the photosensitive element 72, and        a buffer capacitor 66.

The driver 50 with the EL light source 68 operates as follows. Changesin the luminescence brightness as an optical characteristic of the ELlight source 68 are detected by a photosensitive element 72 and fed tothe input of the voltage-controlled generator 54 through a feedbackcircuit comprising the EL light source 68, the photosensitive element72, and the optical signal amplifier 52. The photosensitive element 72may be a photo-resistor, a photo-diode, and/or a photo-transistor.

In a preferred embodiment, the EL light source is a single monolithiccomponent, such as a light wire. In an alternative embodiment, the ELlight source 68 is an integral combination of components, and includes aprincipal light source 74 and an auxiliary light source 76, each ofwhich generates light. The light sources 74, 76 may be arranged andconnected in parallel. The photosensitive element 72 is directed towardsthe auxiliary light source 76 to detect and sense the radiation of ELlight therefrom, and to generate the feedback signals to the amplifier52.

The voltage-controlled generator 54 generates rectangular pulses with anoff-duty factor; that is, the duration of the interval betweenrectangular pulses, which is proportional to the input voltage, forexample, in a relation in which a decrease in the input voltage causes adecrease in the off-duty factor. Rectangular pulses from thevoltage-controlled generator are fed to the control circuitry 56, whichfeeds DC voltage to the input of the DC-to-AC inverter 58 in such a waythat during the supply of a rectangular pulse to the control circuitry56, direct-current voltage is supplied to the input of the DC-to-ACinverter 58, whereas during intervals between the pulses, thedirect-current voltage is not supplied.

Thus, the DC-to-AC inverter 58 generates packets of quasi-sinusoidaloscillations, which are supplied to the EL light source 68 through thebuffer capacitor 66. To improve the shape of the pulse packet, anauxiliary circuit is provided, comprising a logic inverter 64 andauxiliary control circuitry 62. The auxiliary control circuitry 62 is anelectric circuit which commutes the output of the DC-to-AC inverter 58in such a way that the output of the inverter 58 is short-circuitedduring the entire interval between rectangular pulses. Due to thiscircuit, pulse packets contain no characteristic attenuation “tails”associated with free oscillations in the oscillatory circuit constitutedby the inductance of the inverter 58 and the capacitance of the EL lightsource 68. In an example embodiment, a five meter long length of ELwire, commercially available from “LYTEC”, with an electric capacitanceof about 30 nF, may be used as EL light source 68.

The following electronic components are used for producing the drivers50 shown in FIG. 2:

-   -   a) a photosensitive element 72;    -   b) an amplifier 52 of a photosensitive element signal;    -   c) a rectangular-pulse generator 54;    -   d) control circuitry 56;    -   e) a logic inverter 64; f) auxiliary control circuitry 62;    -   g) a DC-to-AC inverter 58; and    -   h) a buffer capacitor 66.

In an example implementation, the photosensitive element 72 is aphotoresistor, such as model number GL-5539 commercially available fromknown sources, which detects radiation and generates a photosensitiveelement signal; the amplifier 52 of the photosensitive element signal isa low power dual operational amplifier, such as model number MC LM358circuit commercially available from “MOTOROLA CORPORATION” or otherknown sources; the rectangular-pulse generator 54 is a model XR4151voltage-to-frequency converter commercially available from “EXARCORPORATION” or other known sources; the control circuitry 56 is a modelnumber MOSFET IRFD 120 circuit commercially available from “MOTOROLACORPORATION” or other known sources; the logic inverter 64 is a modelnumber MC 4093 circuit commercially available from “MOTOROLACORPORATION” or other known sources; the auxiliary control circuitry 62is an MOSFET IRFD 120; the DC-to-AC inverter is a model number IFW5704Db commercially available from known sources; and the buffer capacitoris any known capacitor device having a capacitance of about 30 nF at arating of about 200 V.

FIGS. 3–5 show rectangular pulse diagrams at the input of the controlcircuitry 56 at the beginning, in the middle, and at the end,respectively, of the operating cycle of the EL light source 68, with therectangular pulses displayed relative to a base line 70, representing azero voltage or an offset voltage level. The rectangular pulse in FIG. 5has a continuous value above the base line 70, so the base line 70 inFIG. 5 is represented by phantom lines indicating that the base line 70is not a part of the rectangular pulse in FIG. 5.

FIGS. 6–8 show packets of quasi-sinusoidal pulses at the output ofDC-to-AC inverter 58 at the beginning, in the middle, and at the end,respectively, of the operating cycle of the EL light source 68, witheach of the pulses in FIGS. 6–8 being respectively modulated by therectangular pulses of FIGS. 3–5, respectively. As shown in FIG. 3, atthe beginning of the operating cycle, the duration of an intervalbetween rectangular pulses is longer than that of a pulse, for example,a pulse duration in the beginning of the operating cycle in the presentembodiment is about 3 msec, while interval duration is about 7 msec.With such pulse and interval durations, the quasi-sinusoidal signal issupplied to the EL light source 68 during about 3/10 of the duration ofthe operation of the EL light source 68. The required initial brightnessis ensured by the choice of a suitable frequency and voltage.

In an example embodiment of the present invention, the initial frequencyof about 6,000 Hz and a voltage of about 110 V at the input of the ELlight source 68 ensured substantially the same initial brightness as inthe case of a continuous power supply by a standard inverter with theinitial parameters of the output signal of about 2000 Hz/100 V. With theaging of the EL light source 68, the interval between rectangular pulsesshortens, as shown in FIGS. 3–5, and, respectively, the duration ofsinusoidal signal supply to the EL light source 68 increases, as shownin FIGS. 6–8. At the end of the operating cycle, the rectangular pulseseffectively merge, and the intervals disappear, as shown in FIG. 5, sothat quasi-sinusoidal pulses as shown in FIG. 8 are continuouslysupplied to the EL light source 68.

The circuit of the driver includes a buffer capacitor 66 connected inseries with EL light source 68, thus constituting a divider of voltagepotential. With the aging of the EL light source 68 and, respectively,with an increase in its impedance, the voltage drop on this voltagepotential divider is redistributed in such a way that the voltage on theEL light source grows, which leads to an increase in the lifetime of theEL light source 68.

In an example embodiment of the present invention, the initialcapacitance of the EL light source is about 30 nF, and the buffercapacitance is also equal to about 30 nF. During the operating cycle,the capacitance of the EL light source 68 decreased down to about 22 nF.While in the beginning of the operating cycle, the voltage on the ELlight source 68 is about 110 V, by the end of the operating cycle, thevoltage on the EL light source 68 reaches about 125 V.

EXPERIMENTAL RESULTS

FIG. 9 shows brightness dependencies on the luminescence time for threeidentical EL light sources, which measure brightness changes in units ofpercentage against time measured in hours. Curve 78 relates to an ELlight source fed with sinusoidal oscillations with constant frequencyand amplitude. Curve 80 relates to an inverter-operated EL light source,with the electrical schematic diagram of the inverter in the prior artpresented in FIG. 1. Curve 82 relates to an EL light source operationwith the driver of the present invention. In the three cases, theinitial brightness of EL light sources was identical at about 100 Cd/m².

It is obvious from FIG. 9 that the lifetime 82 of the EL light sourceoperation of the present invention is greatly improved over the priorart operations, shown as curves 78, 80. The lifetime represented by thecurve 82 is longer than the lifetimes represented by the curves 78, 80,such as being approximately three-fold longer than in curve 80 and about4.5-fold longer than in curve 78; for example, when the brightness hasdropped to about 40%.

As shown in FIG. 10, prototypes of the EL light source of the presentinvention were tested and compared to convention light sources overabout 560 hours of continuous operation, with data for the intensity inCd/m² against hours listed as shown in Table 3.

TABLE 3 Light Output Unit ID Maintenance System Number A B C D (%)Conventional 1754 112.69 105.72 91.60 60.79 53.94 Prototype 1749 45.4957.29 58.90 64.73 142.29 Prototype 1750 59.93 78.51 80.10 103.8 173.20Prototype 1752 75.66 87.16 87.27 88.57 117.07 Prototype 1753 64.66 67.8177.28 86.75 134.17in which column A represents the intensity in Cd/m² after 100 hours,being an average of three data points of measurement; column Brepresents the intensity in Cd/m² after 200 hours, being an average ofthree data points of measurement; column C represents the intensity inCd/m² after 300 hours, being an average of three data points ofmeasurement; and column D represents the intensity in Cd/m² after 560hours, based on one data point of measurement. The light outputmaintenance measures the ratio, as a percentage, of the intensity of agiven test device at 560 hours to the intensity of the given test deviceat 100 hours.

A further conventional system having unit ID number 1748 presentedanomalous intensity readings after only about 120 hours. Its performanceis also presented in FIG. 10. The EL light sources of the presentinvention, as demonstrated by operational prototypes, clearly providesuperior intensity during both short and long periods of operationcompared to conventional light sources, even over relatively shortdurations, such as over about 560 hours.

While the preferred embodiment of the present invention has been shownand described herein, it will be obvious that such embodiment isprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those skilled in the art without departingfrom the invention herein. Accordingly, it is intended that theinvention be limited only by the spirit and scope of the appendedclaims.

1. An electroluminescent (EL) device comprising: an EL light source; anda driver, wherein the driver includes: a DC-to-AC inverter forgenerating a high-frequency sinusoidal signal; and a generator oflow-frequency rectangular pulses; wherein the DC-to-AC inverter and thegenerator are connected to feed the sinusoidal signal to the EL lightsource in packets with a duration equal to a duration of the rectangularpulses; and wherein the duration of intervals between the rectangularpulses, with no voltage supplied to the EL light source, is determinedby changes in optical characteristics of the EL light source during thecourse of operation.
 2. The electroluminescent device according to claim1, wherein the driver includes: an electric circuit for shorting theoutput of the DC-to-AC inverter during the entire period of the intervalbetween the rectangular pulses.
 3. The electroluminescent deviceaccording to claim 1, further comprising: a buffer capacitor connectedin series with the EL light source.
 4. The electroluminescent deviceaccording to claim 1, wherein the EL light source is an EL wire.
 5. Theelectroluminescent device according to claim 1, further comprising: anoptical feedback circuit including: a photosensitive element fordetecting radiation from the principal EL light source, and forgenerating a photosensitive element signal; and an amplifier foramplifying the photosensitive element signal; wherein the EL lightsource and the rectangular-pulse generator are connected via the opticalfeedback circuit to receive and to be controlled by the amplifiedphotosensitive element signal.
 6. The electroluminescent deviceaccording to claim 5, wherein the photosensitive element is aphotoresistor.
 7. The electroluminescent device according to claim 5,wherein the photosensitive element is a photo-diode.
 8. Theelectroluminescent device according to claim 5, wherein thephotosensitive element is a photo-transistor.
 9. The electroluminescentdevice according to claim 5, wherein the EL light source includes: aprincipal EL light source; and an auxiliary EL light source enclosed ina driver casing housing the driver, with the auxiliary EL light sourceradiating light and being connected in parallel with the principal ELlight source, with the radiation of light from the auxiliary EL lightsource being sensed by the photosensitive element.
 10. Anelectroluminescent (EL) device comprising: an EL light source; and adriver having: a rectangular-signal generator for generating amodulating rectangular signal with intervals between rectangular pulses;and a modulated-signal generator, responsive to the modulatingrectangular signal, for generating a pulse-width modulatedhigh-frequency quasi-sinusoidal signal fed to the EL light source;wherein the duration of the intervals in the modulating rectangularsignal is determined by the change in optical characteristics of the ELlight source, which are detected by a photosensitive element and fedthrough a feedback circuit to an input of the rectangular signalgenerator to control the generation of the modulating rectangularsignal.